In the course of processing molten materials, it is often necessary to transfer the molten materials from one vessel to another or to circulate the molten materials within a vessel. Pumps for processing molten materials are commonly used for these purposes. The pumps can also be used for other purposes, such as to inject purifying gases into the molten materials being pumped.
This invention relates to equipment for melting metal solids in a furnace, and to a method of melting metal solids in a furnace. More particularly, this invention relates to a molten metal pump facilitating the same.
In the non-ferrous metals industry, scrap recycling has become a way of economic life. In fact, long before environmental concerns and conservation began to drive scrap recycling efforts, recycling of aluminum, copper, zinc, lead and tin occupied a firm niche in the marketplace.
It is known to provide a holding portion of a furnace in which a body of molten metal is heated within an enclosure within which controlled combustion inhibits oxidation of the molten metal. Metal solids are introduced in a well annexed to the holding portion of the furnace and molten metal is transferred between the holding portion and the well in order to both maintain the temperature of the metal in the well and to deliver fresh metal to the holding portion. A molten metal pump is typically used to circulate the metal.
In the aluminum recycling industry in particular, refining processes are complicated greatly by the potency of aluminum to oxidize quite readily. Consequently, refining by oxidating reactions alone, common for other non-ferrous metals, is not feasible. Similarly, aluminum has exceptionally strong alloying characteristics with a variety of other metals, therefore, a broad range of metallic impurities must often be removed during processing. Along these lines, the removal of magnesium has become a particular focus within the industry. The ability to remove magnesium from molten aluminum is made possible by a favorable chemical reaction between magnesium and chlorine. The reaction of the molten aluminum with chlorine ultimately results in the formation of magnesium chloride which collects as a dross on the surface of the molten aluminum in the furnace and can be skimmed away. Although the molten aluminum must be treated, the ultimate goal in the aluminum cast house is to maintain and/or continuously improve product quality while pushing the production rate upward.
As generally outlined above, the secondary production of aluminum alloys often requires the use of a reactive gas to lower magnesium content and/or an inert gas to remove inclusions and hydrogen. Moreover, in order to achieve a desired final magnesium specification for the materials being processed, magnesium removal must occur during the melt refining process. In many operations today, gas injection pumps are considered the most effective tool for this task. Gas injection pumps of this type are depicted in U.S. Pat. Nos. 4,052,199 and 4,169,584, herein incorporated by reference.
Generally, those skilled in the art determine the effectiveness of reactivity by assessing the amount of chlorine that can be introduced into the molten aluminum per unit time. In this context, the maximum amount of chlorine solubilized in the molten aluminum per unit time is readily determinable because aluminum chloride gas which is not reactively scavenged by the magnesium evolves to the surface and decomposes to hydrogen chloride which is visible as a white vapor when in contact with air. Under extremely poor reaction conditions, chlorine itself may not be scavenged by the aluminum and can also be directly emitted from the bath. Given the potential for environmental damage and the hazardous nature of chlorine and hydrogen chloride gases, such results are highly undesirable.
Accordingly, commercial gas injection pumps are operated at a level to prevent such emissions. The primary mechanism for increasing the quantity of chlorine reacted and the corresponding rate at which the magnesium level is reduced, was to operate the pump at higher speeds. Of course, this proves very stressful on the dynamic components of the pump.
Various attempts have been made in the past to modify the discharge component of molten metal pumps. For example, U.S. Pat. No. 5,993,728 discloses the utilization of a convergent nozzle positioned in the outlet passage. Notwithstanding certain advantages provided by this design, the present invention is directed to an alternative approach wherein no or little restriction of the molten metal pathway is introduced.
Alternatively, as shown in U.S. Pat. No. 5,662,725, herein incorporated by reference, a gas-release device is shown. The gas-release device is preferably a rectangular graphite block. The block has a top surface, which is preferably planar or stepped, with an inlet bore formed therein. The inlet bore is preferably threaded and has an inside diameter dimensioned to threadingly receive external threads of a gas-injection tube. The inlet bore extends into the block. A passageway is formed through a side of the block. The passageway communicates with the inlet bore and is preferably cylindrical. A plug is provided, which is preferably formed of graphite, and is received in the passageway at the side to form a gas-tight seal.
Two outlet bores are formed and extend through the block to communicate with a passageway. The outlet bores are preferably cylindrical and are formed at a 0-60, and most preferably at a 45 downstream angle. The term downstream refers to that portion of the molten metal stream that has exited an outlet port and has passed the gas-release device and a 0 downstream angle means that the bore has no downstream angle. In other words, a 0 downstream angle means that the bore(s) is formed perpendicular to the flow of the molten metal stream and releases gas straight up into the stream. A 90 downstream angle, therefore, describes a bore(s) formed in a direction parallel to the direction that the stream flows.
The most preferred positions of the known gas-release block are adjacent the bottom of the outlet port when used in relation to a gas-release device positioned below the center of the outlet port. Accordingly, the gas-release device is positioned so as to not block the outlet port and restrict the flow of molten metal. However, drawbacks to such gas-release device is a larger diameter than the outlet port of the pump base; molten metal entering the device effectively is impeded by the mass of metal therein. This results in the pounding of flowing metal into the device, causing excessive vibrational stresses.
In the case where a molten material is melted in a reverbatory furnace, the furnace is typically provided with an external well in which a pump is disposed. When it is desired to remove molten materials from the vessel, a transfer pump is used. When it is desired to circulate molten materials within the vessel, a circulation pump is used. When it is desired to modify molten materials disposed within the vessel, a gas injection pump is used.
In each of these pumps, a rotatable impeller is disposed within a cavity or housing of a base member that is immersed in a molten material. Upon rotation of the impeller, the molten material is pumped through an outlet or discharge opening and processed in a manner dependent upon the type of pump being used. The impeller itself is supported for rotation in the base member by a rotatable shaft. The shaft is rotated by a motor provided at the shaft's upper end. Several support posts extend from a motor support platform to the base member for supporting and suspending the base member within the molten material. In addition, risers may extend upward from the base member for providing a path or channel for the molten materials to exit through.
Although pumps of the foregoing type have been in effective operation for several years, they still suffer from a variety of shortcomings. For example, graphite or ceramic (i.e. refractory materials) are typically the materials used for constructing many of the components of pumps used for processing molten materials because of its low cost, relative inertness to corrosion, and its thermal shock resistance. Although graphite has advantages when used for certain components of molten material pumps, it is not the most advantageous material to be used for complicated shapes and mechanically stressed components.
Various attempts have been made in the past to modify the discharge component of molten metal pumps. For example, in U.S. Pat. No. 5,993,728 discloses the utilization of a convergent nozzle positioned in the outlet passage. Notwithstanding certain advantages provided by this design, the present invention is directed to an alternative approach wherein no restriction of the molten metal is introduced.
Rather, it is preferable to make these types of components, e.g. support posts, risers and rotating shafts, include a metallic material, such as iron based alloys or steel, since metallic materials are considerably stronger per pound than graphite. The problem with using these materials is that the base member and impeller are typically constructed from graphite (due to its wear characteristics) and it is difficult to maintain a connection between metallic and graphite components. Such a difficulty arises because of the differences in thermal expansion experienced by these materials. Accordingly, bolts and conventional fasteners are generally not feasible connecting mechanisms.
Known connections between the support posts and the motor support platform do not allow for easy adjustments to facilitate leveling of the motor support platform. An unleveled motor support platform can cause stress on many of the components of the molten metal pump.